Communication devices such as wireless devices are also known as e.g. user equipments (UE), mobile terminals, wireless terminals, and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an network node. A cell is the geographical area where radio coverage is provided by the network node.
The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each network node may support one or several communication technologies. The network nodes communicate over the air interface operating on radio frequencies with the wireless terminals within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission in LTE is controlled by the radio base station. Upon the demand on higher bandwidth and higher data rate applications, LTE-Advanced as of 3GPP Release 10 introduces Carrier Aggregation (CA).
Carrier Aggregation allows expansion of effective bandwidth delivered to a wireless device through concurrent utilization of radio resources across multiple carriers. This means that several Component Carriers (CCs) may be aggregated to form a larger overall transmission bandwidth. A wireless device that is CA capable may be configured with multiple component carriers, corresponding to multiple serving cells, from the same network node. Then, the wireless device with reception and/or transmission capabilities for carrier aggregation may simultaneously receive and/or transmit on these multiple component carriers originating from the same network node. Carrier Aggregation is supported for both contiguous and non-contiguous component carriers.
It is possible to configure a wireless device to aggregate a different number of component carriers originating from the same network node and of possibly different bandwidths in the UL and the DL. The number of DL component carriers that may be configured depends on the DL aggregation capability of the wireless device. The number of UL component carriers that may be configured depends on the UL aggregation capability of the wireless device. Component carriers originating from the same network node need not to provide the same coverage.
As mentioned above, each component carrier corresponds to a serving cell. Thus, the wireless device may have multiple serving cells, each serving cell operating on a respective component carrier.
The wireless device may connect to one primary serving cell (PCell), served by a Primary Component Carrier (PCC), originating from a serving network node, such as an eNB. The primary serving cell may also be referred to as a primary cell. Additionally, the wireless device may also connect to one or several secondary serving cells (SCells). The secondary serving cells may also be referred to as secondary cells. Each secondary cell is served by a corresponding Secondary Component Carrier (SCC), originating from the same network node as the PCC. The PCC may be regarded as the anchor carrier for the wireless device and is thus used for basic functionalities such as radio link failure monitoring. The Radio Resource Control (RRC) connection is handled by the secondary cell, served by the PCC. After connection establishment, one or several secondary cells may be configured and activated, to provide additional radio resources.
The secondary cells are added and removed as required, while the primary cell is only changed at handover. If a component carrier is activated, it may be used for data transmission to the wireless device. Configured component carriers are candidates for activation. That is, based on the arrival of a significant amount of data, one or more of the configured component carriers may be activated.
Via RRC signaling, the wireless device may indicate to a serving network node its capability to support one or more secondary cells in the downlink, as well as one or more secondary cells in the uplink. The serving network node may alternatively receive information from a different network node about the capability of the wireless device to support one or more secondary cells.
In many scenarios, the configuration of the wireless device and the actions taken by the network node are dependent on a level of coverage of the component carriers available for the wireless device. The network node may establish a level of coverage of a component carrier, and the serving cell served by that component carrier, based on measurements performed by the wireless device.
Typically the wireless device continuously performs intra-frequency measurements, to always search for the most suitable serving cell. Inter-frequency measurements are generally only performed when explicitly needed. The main reason for this is that inter-frequency measurements normally require measurement gaps. A measurement gap is a period during which no transmission and reception happens. Measurements requiring measurement gaps may affect the performance of the wireless device as well as battery consumption. Continuous use of measurement gaps may also increase the drop rate.
With the introduction of carrier aggregation there is an increased need for inter-frequency measurements to find suitable secondary cells. In case no suitable secondary cell is available, these measurements only decrease the performance of the wireless device, to no gain.